Slot line tunable bandpass filter

ABSTRACT

A tunable bandpass filter comprising a pair of slot line resonant cavities formed in a metallic film covering the planar surface of a ferrite substrate. A magnetic biasing field of variable magnitude is generated and applied through the ferrite substrate transversely across the slot line resonant cavities. The magnetic field operates to change the electrical length of the resonant cavities due to the fact that the magnetic permeability of the ferrite material changes with the applied field.

The invention described herein may be manufactured and used by or forthe Government for governmental purposes without the payment of anyroyalties thereon or therefor.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to microwave and millimeter wave signalfilters and more particularly to such filters which utilize slottransmission lines as the propagating medium.

2. Description of the Prior Art

Slot line filters and the characteristics of slot transmission lines arewell known and described, for example, in: IEEE Transactions OnMicrowave Theory And Techniques, "Slot Line Characteristics", Elio A.Mariani, et al., Vol. MTT-17, No. 12, December, 1969, pp. 1091-1096; andIEEE Transactions On Microwave Theory And Techniques, "Slot-Line FiltersAnd Couplers", Elio A. Mariani, et al., Vol. MTT-18, No. 12, December,1970, pp. 1089-1095. As disclosed in those two articles, a slottransmission line comprises an elongated, relatively narrow slot formedin a metal coating applied to a dielectric substrate. This structureprovides a transmission line for application to microwave integratedcircuits. The slot line offers a unique combination of a planar typegeometry and a TE dominant mode similar to the dominant mode of arectangular waveguide. Furthermore, the slot line can be combined withmicrostrip circuitry utilizing a common dielectric substrate whereby twotransmission lines are coupled through the dielectric medium.

Where a filter configuration is desired, the article entitled, "SlotLine Filter Couplers", proposes configuring a bandpass filter, forexample, by a pair of quarter wave coupled resonant slots or a pair ofend coupled resonant slots fabricated on a dielectric substrate.Coupling to and from the resonant slots can be achieved by either of twomeans: using a grounded center conductor of a semi-rigid coaxial cabletraversing the slot and forming a magnetic coupling loop thereby orusing a length of microstrip transmission line traversing the slot onthe opposite side of the substrate.

Where a tunable filter is desired, the current state of the art proposesthe use of the well known magnetically-tunable, yttrium-iron-garnat(YIG) filter; however, such structures result in relatively complex,costly coaxial circuit configurations which are non-planar.

Accordingly, it is an object of the present invention to provide animprovement in slot transmission line filters.

It is a further object of the invention to provide a tunable slottransmission line filter.

It is yet a further object of the invention to provide a tunable slottransmission line filter which provides relatively narrow-band frequencyselectivity, i.e. pass-band, over a relatively broad operating band inthe microwave and millimeter wave frequency range.

SUMMARY

Briefly, the foregoing and other objects are achieved by a pair of slotline resonant cavities formed in a metallic film covering a generallyflat major face of a ferrite substrate. The two resonant cavities have alength of one half the guide wavelength and are end-coupled in a firstembodiment and quarter-wavelength, side-coupled in a second embodiment.The mutual separation between the slots determines the coupling strengthbetween the coupled slots and in turn determines the actual bandwidth ofthe filter. Frequency tunability is provided by the application of avarying biasing magnetic field through the ferrite substratetransversely to the longitudinal dimension of the slots, whoseelectrical lengths are effectively changed as a result of the change inpermeability of the ferrite material as a function of the applied field.Microwave/millimeter wave energy is coupled to and from the filter byway of a magnetic coupling loop formed using either the center conductorof a coaxial transmission line shorted to the metallic film or by way ofan open microstrip transmission line orthogonally located on theopposite side of the substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view generally illustrative of a firstembodiment of the invention;

FIG. 2 is a schematic diagram illustrative of one known method ofgenerating and applying a magnetic biasing field to the substrate shownin FIG. 1;

FIG. 3 is a perspective view of a second embodiment of the invention;

FIG. 4 is a perspective view generally illustrative of a thirdembodiment of the invention; and

FIG. 5 comprises an end view of the embodiment shown in FIG. 4.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Whereas known slot line bandpass filters employ dielectric substrates ofrelatively high permittivity ε_(r), a typical example being magnesiumtitanate where ε_(r) is equal to 13, such devices provide a fixedfrequency characteristic. The present invention, on the other hand, isdirected to a variable slot line bandpass filter having a tunablefrequency characteristic. This is achieved in a first embodiment, asshown in FIG. 1, by a pair of resonant cavities in the form of two equallength elongated rectangular slots 10 and 12 etched, for example, in arelatively thin metallic film 14 affixed to a substantially flat majori.e. relatively large upper face 16 of a substrate 18 comprised offerrite material and having a generally rectangular cross sectionincluding a bottom major face 20 and a pair of opposing side faces 22and 24. The metallic film 14 is preferably comprised of copper. In theconfiguration shown in FIG. 1, the two slots 10 and 12 have a relativelynarrow width and a length of λ_(g) /2 where λ_(g) represents the guidewavelength, the wavelength of the electromagnetic wave energy in thefilter or wave guide.

The resonant cavities 10 and 12 of FIG. 1, moreover, are shown in anend-coupled configuration and having a mutual separation S whichdetermines the coupling strength between the coupled cavities. This inturn determines the fractional bandwidth (Δf/f₀) of the filter, where f₀is the center frequency of the pass band, and ΔF is the bandwidth ofoperation.

Input and output coupling of microwave/millimeter wave energy isprovided by a pair of coaxial cable transmission lines 26 and 28, thecenter conductors 30 and 32 of which respectively traverse the resonantcavities 10 and 12 and which are soldered to the outer surface andeffectively is shorted to the metallic film 14, thus forming a magneticcoupling loop.

The bandpass filter thus configured is made tunable to provide arelatively narrow band frequency selectivity over the broad operatingrange by the application of a selectively variable biasing magneticfield H through the ferrite substrate across its width dimension so asto be mutually orthogonal to the length dimension of the resonant cavityslots 10 and 12. By providing a magnetic bias, H, the electrical lengthsof the resonant cavities 10 and 12 are effectively changed since thepermeability of the ferrite material changes with the applied field.

The biasing magnetic field H can be generated in any conventionalmanner; however, an illustrative means is shown in FIG. 2 whichcomprises an electromagnet comprised of an open core 34 having a pair ofpole faces 36 and 38 placed adjacent the side faces 22 and 24 of theferrite substrate 18. An electrical coil 40 is wound around the core 34and a source of variable DC voltage 42 is connected thereto, thusproviding a variable magnetic field H through the body of the ferritesubstrate 18 as shown in FIG. 1. Thus by changing the effectiveelectrical length of the resonant slots 10 and 12 shown in FIG. 1, ashift in the filter operating frequency is effected due to the fact thatthe slot length determines the resonant or center frequency of thebandpass filter.

A second embodiment of the invention is shown in FIG. 3. This embodimentis illustrative of a quarter-wavelength, side-coupled configuration.There, as shown, the pair of slots 10 and 12 formed in the metallic film14 comprise equal length half-wavelengt λ_(g) /2 cavities which areparallel and offset from each other by a separation S, but whoserespective length dimensions mutually overlap by a quarter-wavelengthλ_(g) /4. Input and output coupling in the embodiment shown in FIG. 3 isnow provided, however, by a pair of open microstrip transmission lines44 and 46 arranged orthogonally to the slots 10 and 12 on the oppositemajor face, i.e. bottom surface 20 of the ferrite substrate 18. As inFIG. 1, a biasing magnetic field H is applied through the body of theferrite substrate 18 transversely across the resonant slots 10 and 12.When desirable, input and output coupling to the configuration shown inFIG. 3 can be provided by coaxial cable techniques shown in FIG. 1 andvice versa.

Both filter configurations, moreover, can be fabricated using a lowcost, precision photolithographic process whereby the slot line circuitas well as the microstrip lines could employ this type of fabrication.Use of photolithographic fabrication, moreover, permits low costmonolithic devices having precision dimensions to within a tolerance of±0.0005 inches.

Another embodiment contemplated by the subject invention is shown inFIGS. 4 and 5 and is similar to the configuration shown in FIG. 1 withthe exception that a toroidal ferrite substrate 18' is utilized. As inFIG. 1, a pair of end coupled resonant slots 10 and 12 are formed in themetallic film 14 fabricated on the top surface 16 and input and outputcoupling is achieved by way of a magnetic coupling loop formed using thecenter conductors 30 and 32 of a pair of coaxial cables 26 and 28. Thetoroidal ferrite substrate includes a central axial opening 48conveniently shown having a rectangular cross section. The opening 48permits the ferrite substrate 18' to be magnetized by a current carryinglatching wire 50 which passes interiorly through the opening 48 alongits entire length where it is connected, for example, across a variableDC source 42. With the current I flowing in the direction shown, themagnetic field H will have a polarity as shown in FIG. 5. This toroidalconfiguration allows the ferrite to be magnetized to a given state; thatmagnetization state is then retained without a holding current in thecircuit. Such a filter as well as the others shown in FIGS. 1 and 3 areparticularly adaptable to operate as a tunable pre-selector forcommunications or electronics receiver front-ends operating in themicrowave/millimeter wave frequency bands.

It should also be noted that the "shorted" coaxial or "open" microstriptransmission line input and output means are not restricted to thespecific embodiments shown in FIGS. 1 and 3, respectively, but each typecan be applied to either the end-coupled or the quarter-wave coupledresonator configuration depending on the needs of the circuit designer.

Having thus shown and described what is at present considered to be thepreferred embodiments of the invention, it should be noted that the samehas been made by way of illustration and not limitation. Accordingly,all modifications, alterations and changes coming within the spirit andscope of the invention as defined in the appended claims are hereinmeant to be included.

We claim:
 1. A tunable bandpass filter for microwave and millimeter wavefrequencies, comprising:a substrate of ferrite material and having atleast one generally flat planar face; a relatively thin layer ofmetallic material having a pair of intercoupled resonant slots thereinlocated on said planar face, each of said slots having an elongatedlength and a relatively narrow width; means for providing an input ofmicrowave and millimeter wave signals to one of said pair of resonantslots; means for providing an output of microwave and millimeter wavesignals from the other of said pair of resonant slots; and means forgenerating and applying a biasing magnetic field of predeterminedintensity through said substrate substantially orthogonal to the lengthof each of said pair of resonant slots and parallel to said planar facewhereby the permeability of the ferrite material changes as a functionof the intensity of the applied magnetic field to thereby change theeffective length of said resonant slots and accordingly the resonantfrequency of the filter.
 2. The filter as defined by claim 1 whereineach of said slots has a length equal to one-half the wavelength of theelectromagnetic wave energy in the filter.
 3. The filter as defined byclaim 2 wherein said pair of slots are mutually linearly aligned andhaving a predetermined separation to provide an end coupled slotconfiguration and where the separation between the slots determines thecoupling strength between the coupled slots.
 4. The filter as defined byclaim 2 wherein said pair of slots are parallel and are mutuallyside-by-side for substantially one-half of their respective lengths toprovide a quarter-wavelength side-coupled configuration, and beingmutually separated by a predetermined distance which determines thecoupling strength between the coupled slots.
 5. The filter as defined byclaim 2 wherein said substrate is of a generally rectangular crosssection and including a pair of relatively large top and bottom majorplanar faces and a pair of relatively smaller side minor faces, whereinsaid layer of metallic material is located on said top major planar faceand said biasing magnetic field is applied through said side faces. 6.The filter as defined in claim 5 and wherein said means for generatingsaid biasing magnetic field includes means for varying the intensity ofsaid magnetic field.
 7. The bandpass filter as defined by claim 1wherein said thin layer of metallic material comprises a film of copperformed on said planar face.
 8. The filter as defined by claim 1 whereinsaid means for providing an input and an output of signal energy to andfrom said pair of resonant slots comprise a pair of coaxial transmissionlines, each including a center conductor traversing said resonant slotsand being connected to said layer of metallic material on one of thelonger sides of said slots to form thereby magnetic coupling loops. 9.The filter as defined by claim 1 wherein said means for providing aninput and an output of signal energy to and from said pair of resonantslots includes a pair of microstrip transmission lines alignedorthogonally relative to the lengths of said resonant slots on a secondplanar face of said substrate opposite said at least one planar face.10. A tunable bandpass filter for microwave and millimeter wavefrequencies comprising:a continuous toroidal ferrite substrate havingtop and bottom exterior planar faces; a layer of metallic materialhaving a pair of intercoupled resonant slots therein located on one ofsaid planar surfaces, each of said slots having an elongated length andrelatively narrow width; means for providing an input of microwave andmillimeter wave signals to one of said pair of resonant slots; means forproviding an output of microwave and millimeter wave signals from theother of said pair of resonant slots; and means for generating atransverse magnetic flux in said one planar surface of said toroidalsubstrate normal to the length of each of said slots and parallel tosaid one planar surface.
 11. The filter as defined by claim 10 whereinsaid means for generating includes an electrical conductor passingthrough said toroidal substrate.